1. Field of the Invention
The present invention relates to a fuel pump control apparatus for controlling the operating speed of a fuel pump which is used, for example, with a vehicle engine, so as to force the fuel to be supplied to the fuel injectors of the engine.
2. Description of the Prior Art
In the past, a fuel pump control apparatus of this type has been disclosed in Japanese Unexamined Publication No. 60-147563.
The fuel pump control apparatus disclosed in the above publication is constructed so that the operating speed of a fuel pump is changed from one speed to another in accordance with the quantity of fuel injected per unit time from each fuel injector and the intake air pressure. This has the effect of not only producing merits such as the elimination of wasteful operation of the fuel pump, a reduction in the pump operating sound during the idling period and an improvement in the pump life, but also preventing the quantity of fuel supplied to the fuel injectors from the fuel pump from becoming less than the quantity of fuel injected, thereby preventing the danger of causing any undersupply of the fuel.
However, since the fuel pump is deteriorated in performance as it is used over a long period of time, even if the applied voltage to the motor for the fuel pump is the same, the rotating capacity of the motor is deteriorated so that after a long period of use, the operating speed of the fuel pump is decreased as compared with the initial operating speed and therefore the quantity of fuel supplied to the fuel injectors is decreased.
When the deterioration in the performance of the fuel pump proceeds in this way, where the fuel pump is controlled to operate at a low speed and moreover the fuel pump is being operated in a condition close to a level at which the switching from the low speed to the high speed takes place, there is a disadvantage that in contrast to the performance during the initial use period which is capable of supplying the fuel in an amount which has some margin over the required fuel injection quantity, the reduced fuel supply quantity due to the reduced operating speed gives rise to a situation in which the quantity of fuel supplied is less than the quantity of fuel injected and the engine is not operated smoothly.